It is not common, but I suppose it is in itself normal. The cord is supposed to dry out and fall off if nothing else happens with it. So is with human babies too as I remember.
Good luck!
Ps. How is this kitten otherwise? does it seems to gain properly? Or it is this kitten you are anxious about?
While taking a little break, it struck me, it happens if the intestine hangs out, people if not very experienced think it is the cord...
In such a case there is not much to do. A very skilled vet surgeon may have a try, but most are pts.